Interesting one spanning 2 months, but second number is the same.
Yeah forgot those, maybe because they are so rare but IIRC (maybe not) ATO mentioned one of those earlier or elsewhere, or it was you idk... and I have one too (1981, NST: 11=5 and BST: 12=5)... but that means that...
So the "month" numbers go into the guitars only during final production and (speculation) are therefore more connected to the lot number than anything else. In other words, if the lot number refers to an order, it would indeed confirm the "n-th order of the month X" theory.
... isn't quite right (or just plain wrong). Let's still assume "lot" means order:
The parts were obviously stamped before they were sent over to the paint shop (at least that's what we deduce from the stamps having color codes). That means the workers who finished the parts to that point knew for what order they are. In my example (#1024464), the neck was ready for spraying let's say Monday, November 30th, so it got the 11=5, the body was finished only Tuesday, December 1st and therefore got the 12=5.
So for which month was that order #5? Obviously for November, right? Hm...1981, the year of "monthly order numbers" going up to (known) 13 in summer, not sure if Christmas increases demand for replica guitars... and they were processing order #5 so late that the the body got sprayed in December. Could be, we don't know how much backlog they had... but apart from the one =35, 40, 42 outliers in 1978, the lot numbers rarely go beyond 20 and that would mean they had at best 20 different customers
worldwide per month after 1978? Doesn't that sound unlikely?
That's where "lot" as in "lot" could make (maybe even a lot of) sense: Customer #10 wants 20 ST50s, customer #16 wants 30, customer #31 orders 40, customer #1 wants 100 ST42s and everyone wants 10 ST60s and 5 ST80s... that's where you could bundle the production of the ST42s and ST50s to a "lot" and the production of the ST60s and 80s to another lot. Same goes for all other models, they are produced in bigger lots (just another word for "batches", right?), which may explain relatively low lot numbers and what looks like burst-like patterns.
One advantage could be that producing certain models in bigger chunks avoids changing jigs, tools, CNC programs and tool heads 5 times a day etc. The contents (models) within each lot could be distributed differently of course, and I have no idea how the double numbers could fit into this scheme. Studying ATO's list, it actually looks a bit like groups with the same stamps are similar or the same models. We need more samples!
Unfortunately, no factory video and no amount of googling has brought up anything to learn about planning and logistics of industrial guitar mass-production so far.